$ cat foo.pl
package foo;
use strict;
use warnings;
my $x;

tu_was();

sub tu_was {
    my $y = $x + 23;
}

1;

$ perl -wle'
eval { require "foo.pl" };
print "error: <$@>"'
Use of uninitialized value $x in addition (+) at foo.pl line 9.
error: <>